Actually, the experiment worked because of an intelligent intervention. There was no evidence of anything else. I need not prove what is already established.
Yes, humans deliberately applied selection pressures to the fox population. This is called "methodology." The results of the experiment demonstrate that constant selection pressure can and does change the characteristics of a population after a series of generations. This is called "evidence."
The question you have left unanswered is how are selection pressures imposed by an intelligent agent different from undirected selection pressures?